KR101597058B1 - Cold-rolled steel sheet - Google Patents

Abstract

C: not less than 0.020%, not more than 0.30%, Si: not less than 0.10% but not more than 3.00%, Mn: not more than 1.00%, not more than 3.50%, and a high tensile strength steel sheet excellent in ductility, work hardenability, stretch flangeability and high tensile strength of 780 MPa or more. And a metal structure containing residual austenite in the second phase. The residual austenite is the volume ratio is less than 4.0%, greater than 25.0%, an average particle diameter of the entire tissue is less than 0.80μm, the number density of the retained austenite, residual austenite grain diameter is less than 1.2μm 3.0 × 10 ^-2 탆 / ² or less.

Description

Cold rolled steel sheet {COLD-ROLLED STEEL SHEET}

The present invention relates to a cold-rolled steel sheet. More specifically, the present invention relates to a high-strength cold-rolled steel sheet excellent in ductility, work hardenability and stretch flangeability.

BACKGROUND ART At present, as the industrial technology field is highly fragmented, special and high performance is required for the materials used in each technical field. For example, even for cold-rolled steel sheets used by press forming, more excellent formability is required along with the diversification of the press form. Further, high strength is required, and application of a high-strength cold-rolled steel sheet has been studied. Particularly, with respect to steel sheets for automobiles, the demand for high-strength cold-rolled steel sheets with a thin steel sheet is remarkably increased in order to reduce the weight of the vehicle body from consideration for the global environment to improve fuel economy. In press forming, a thinner steel sheet used tends to cause cracks and wrinkles, so a steel sheet having more ductility and elongation flangeability is required. However, such press formability and high strength of the steel sheet are characteristics of betrayal, and it is difficult to simultaneously satisfy these properties.

Heretofore, as a method for improving the press formability of a high-strength cold-rolled steel sheet, many techniques for microfabrication of microstructures have been proposed. For example, Patent Document 1 discloses a method of producing a fine-grain high-strength hot-rolled steel sheet in which rolling is performed at a total rolling reduction of 80% or more at a temperature near the Ar ₃ point in the hot rolling step. Patent Literature 2 discloses a method for producing super fine grain ferrite which continuously performs rolling at a reduction ratio of 40% or more in the hot rolling step.

These techniques improve the balance between strength and ductility in the hot-rolled steel sheet, but there is no description in the patent literature on a method for improving the press-formability by microfilling the cold-rolled steel sheet. According to the investigations of the present inventors, it is difficult to obtain a cold-rolled steel sheet excellent in press-formability because cold-rolled and annealed using the fine-grained hot-rolled steel sheet obtained by the large-reduction rolling as a base material tends to make coarse crystal grains. Particularly, in the production of a composite structure cold rolled steel sheet containing a low-temperature transformation-forming phase or retained austenite in a metal structure which needs to be annealed at a high temperature of Ac ₁ point or more, coarsening of crystal grains during annealing is remarkable, It is impossible to enjoy the advantage of the composite cold rolled steel sheet which is said to stand out.

Patent Document 3 discloses a method of manufacturing a hot-rolled steel sheet having a fine grain by performing a rolling down in a dynamic recrystallization zone in a hot rolling step by a downward stroke of 5 stands or more. However, it is necessary to extremely reduce the temperature drop at the time of hot rolling, and it is difficult to carry out with the ordinary hot rolling equipment. In addition, examples of cold rolling and annealing after hot rolling are disclosed, but the balance between tensile strength and hole expandability (stretch flangeability) is poor and press formability is insufficient.

With regard to cold-rolled steel sheets having microstructure, Patent Document 4 discloses high-strength cold-rolled steel sheets for automobiles having excellent collision safety and moldability, in which residual austenite having an average crystal grain size of 5 탆 or less is dispersed in ferrite having an average crystal grain size of 10 탆 or less . In a steel sheet containing retained austenite in a metal structure, a large elongation is exhibited due to the transformation-induced aging (TRIP) caused by martensitization of austenite during processing. However, due to the formation of hard martensite, do. In the cold-rolled steel sheet disclosed in Patent Document 4, the ductility and hole expandability are improved by making the ferrite and the retained austenite finer. However, it is difficult to say that the hole expanding ratio is at most 1.5 and sufficient press formability is provided. Further, in order to increase the work hardening index and improve the collision safety, it is necessary to make the main phase a soft ferrite phase, and it is difficult to obtain a high tensile strength.

Patent Document 5 discloses a high strength steel sheet excellent in elongation and stretch flangeability in which fine particles of a second phase composed of residual austenite and / or martensite are finely dispersed in crystal grains. However, in order to finely disperse the second phase to nanosize and disperse it in the crystal grains, it is necessary to incorporate a large amount of expensive elements such as Cu and Ni and to conduct a solution treatment for a long time at a high temperature, The productivity is remarkably deteriorated.

Patent Document 6 discloses a high-strength hot-dip galvanized steel sheet in which residual austenite and low temperature transformation forming phase are dispersed in ferrite and temper martensite having an average crystal grain size of 10 탆 or less and excellent ductility, stretch flangeability and endothelial property . Trim martensite is an effective phase for improvement of stretch flangeability and endothelial property, and it is said that these characteristics are further improved by refining tume martensite. However, in order to obtain a metal structure containing temperate martensite and retained austenite, it is necessary to perform primary annealing for producing martensite, secondary annealing for trimming martensite and obtaining residual austenite So that the productivity is greatly damaged.

Patent Document 7 discloses a hot rolled steel sheet which is quenched immediately after hot rolling to a temperature of 720 占 폚 or less and kept at a temperature range of 600 to 720 占 폚 for 2 seconds or longer to perform cold rolling and annealing on the obtained hot rolled steel sheet, A method of manufacturing a cold-rolled steel sheet is disclosed.

Japanese Patent Application Laid-Open No. 58-123823 Japanese Patent Application Laid-Open No. 59-229413 Japanese Patent Application Laid-Open No. 11-152544 Japanese Patent Application Laid-Open No. 11-61326 Japanese Patent Application Laid-Open No. 2005-179703 Japanese Patent Application Laid-Open No. 2001-192768 International Publication No. 2007/15541 pamphlet

The technique disclosed in the above-described Patent Document 7 is characterized in that, after the end of hot rolling, a fine rib structure is formed by ferrite transformation of the work deformation as a driving force without releasing the work deformation accumulated in the austenite and the workability and thermal stability It is excellent in that an improved cold rolled steel sheet can be obtained.

However, with the recent need for new high performance, there is a demand for a cold-rolled steel sheet which has high strength, good ductility, good work hardenability and good stretch flangeability at the same time.

The present invention has been made to meet such a request. Specifically, a problem to be solved by the present invention is to provide a high tensile strength cold rolled steel sheet having excellent ductility, work hardenability, stretch flangeability and tensile strength of 780 MPa or more.

The present inventors conducted a detailed investigation on the influence of the chemical composition and the manufacturing conditions on the mechanical properties of the high-tensile cold-rolled steel sheet. In the present specification, "% " representing the content of each element in the chemical composition of steel means the total mass%.

A series of steels containing not less than 0.020% but not more than 0.30% of Si, more than 0.10% of not more than 3.00%, Mn of more than 1.00% of not more than 3.50%, P of not more than 0.10%, S of not more than 0.010%, sol . Al: not more than 2.00%, N: not more than 0.010%.

The slab having such a chemical composition was heated to 1200 캜 and then hot rolled to a plate thickness of 2.0 mm in various depressions in a temperature range of Ar ₃ point or higher. After hot rolling, the slab was cooled After air cooling for 5 to 10 seconds, the mixture was cooled to various temperatures at a cooling rate of 90 ° C / s or lower, and this cooling temperature was taken as the winding temperature, charged into an electric heating furnace maintained at the same temperature and held for 30 minutes , And the furnace was cooled at a cooling rate of 20 DEG C / h to simulate annealing after winding. The hot-rolled steel sheet thus obtained was pickled and cold-rolled to a plate thickness of 1.0 mm at a rolling rate of 50%. The obtained cold-rolled steel sheet was heated to various temperatures using a continuous annealing simulator, held for 95 seconds, and cooled to obtain an annealed steel sheet.

A test piece for tissue observation was taken from the hot-rolled steel sheet and the annealed steel sheet and subjected to a scanning electron microscope (SEM) equipped with an optical microscope and an electron beam backscattering pattern analyzer (EBSP) , And the volume percentage of retained austenite at 1/4 depth from the surface of the steel sheet of the annealed steel sheet was measured using an X-ray diffractometer (XRD). The tensile test specimens were taken from the annealed steel plate along the direction perpendicular to the rolling direction, and subjected to a tensile test. The ductility was evaluated by total elongation, and the work hardening property was evaluated by a work hardening index (n value) of 5 to 10% . Further, a 100 mm square hole expanding test piece was taken from the annealed steel plate, and a hole expanding test was conducted to evaluate the stretch flangeability. In the hole expansion test, a punching hole having a diameter of 10 mm was punched with a clearance of 12.5%, and a punching hole was enlarged with a conical punch having a tip angle of 60 ° to measure the enlargement ratio (hole expanding rate) .

As a result of these preliminary tests, the following knowledge (A) to (H) was obtained.

(A) Hot-rolled steel sheets produced through a so-called quench-cooling process in which they are quenched by water cooling immediately after hot-rolling, specifically, hot-rolled steel sheets produced by quenching to a temperature range of 720 ° C or lower within 0.40 seconds from the completion of hot- When the steel sheet is rolled and annealed, the ductility and stretch flangeability of the annealed steel sheet improve with the increase of the annealing temperature. When the annealing temperature is too high, the austenite grains coarsen and the ductility and stretch flangeability of the annealed steel sheet sharply It may deteriorate.

(B) Raising the final rolling reduction of hot rolling suppresses coarsening of austenite grains that may occur during annealing at a high temperature after cold rolling. The reason for this is not clear, but (a) the higher the final reduction in load, the more the ferrite fraction is increased in the metal structure of the hot-rolled steel sheet, and (b) (C) Since the ferrite grain boundary serves as a nucleation site in the transformation from ferrite to austenite during annealing, the nucleation frequency increases as the number of fine ferrite increases, and austenite (D) coarse low temperature transformation phase is assumed to be due to coarse austenite grains during annealing.

When the coiling temperature is raised in the winding step after quenching immediately after the step (C), coarsening of the austenite particles which may occur during annealing at a high temperature after cold rolling is suppressed. Although the reason for this is not clear, (a) the hot-rolled steel sheet becomes fine by quenching immediately after the hot-rolled steel sheet, and therefore the precipitation amount of the iron carbide in the hot-rolled steel sheet remarkably increases with the increase of the coiling temperature; (b) (C) the iron carbide which is not solid-dissolved is used as the nucleating site in the transformation from the ferrite to the austenite during the annealing, so that the nucleation rate increases as the deposition rate of the iron carbide increases, It is presumed that the austenite is caused by grain refinement because it suppresses grain growth of austenite.

(D) The greater the Si content in the steel, the stronger the anti-coarsening effect of the austenite grains is. Although the reason for this is not clear, (a) the iron carbide becomes finer and the number density increases as the Si content increases, (b) the nucleation frequency in the transformation from ferrite to austenite And (c) the increase in the iron carbide in the non-solidified steel is further suppressed by the growth of the austenite grains and the austenite is further refined.

(E) cracking and cooling at a high temperature while suppressing coarsening of the austenite grains, the fine phase of the low-temperature transformation is formed as the main phase, and the second phase is formed of the fine retained austenite and, in some cases, a metal structure containing fine polygonal ferrite .

1 is a graph showing the results of the hot rolling at a final reduction of 42% in plate thickness reduction rate, a rolling finish temperature of 900 占 폚, a quenching stop temperature of 660 占 폚, and a time from completion of rolling to quenching of 0.16 seconds, , Which is a graph showing the result of examining the grain size distribution of retained austenite in a annealed steel sheet obtained by cold-rolling a hot-rolled steel sheet and annealing at a cracking temperature of 850 ° C. Fig. 2 is a graph showing the results of examining the grain size distribution of the retained austenite in the annealed steel sheet obtained by hot rolling the slab having the same chemical composition immediately after the hot rolling by the ordinary method, cold rolling and annealing to be. 1 and 2, it can be seen from the comparison of Figs. 1 and 2 that the annealed steel sheet (Fig. 1) produced through a quenching process immediately after the formation of the steel sheet has suppressed the formation of coarse retained austenite grains having a grain size of 1.2 탆 or more and that the retained austenite is finely dispersed know.

(F) By suppressing the formation of coarse retained austenite grains having a grain size of 1.2 탆 or more, the stretch flangeability of the steel sheet having the low-temperature transformation forming phase as the main phase is improved.

3 is a graph showing the relation between TS × λ ^{1 .7} and retained austenite particle size crude number density (N _R) of the nitro to 1.2μm or more. TS is the tensile strength, the hole expanding ratio λ, ^{1 .7} TS × λ is an indicator for evaluating the hole expandability from the balance of the strength and the hole expansion rate. As shown in the diagram, TS ^{1 .7} × λ is know to have a correlation with the N _R, N _R is lower The improved hole expandability. Although the reason for this is not clear, (a) the retained austenite changes to hard martensite by processing, but when the retained austenite grains are coarse, the martensite grains become coarse and the stress concentration becomes high, (B) coarse retained austenite particles are likely to be originated from cracks rather than fine retained austenite particles because they are martensitized at an early stage of processing .

(G) As the annealing temperature rises, the fraction of the low-temperature transformation forming phase tends to increase and the work hardening property tends to deteriorate. By suppressing the formation of coarse retained austenite grains having a grain size of 1.2 탆 or more, It is possible to prevent deterioration of work hardening property in the steel sheet as the main phase.

4 is a graph showing the relationship between the value of TS x n and N _R. The TS x n value is an index for evaluating work hardenability from the balance of strength and work hardening index. As shown in the figure, the value of TS x n has a correlation with N _R, and it is found that the work hardening property is improved as the N _R is lower. Although the reason for this is not clear, (a) coarse retained austenite grains are martensitized at an early stage of processing at a strain of less than 5%, so that the deformation range is almost equal to the increase of n value at 5 to 10% (B) suppressing the formation of coarse retained austenite grains is presumed to be caused by an increase in fine retained austenite grains martensitized at a high strain rate of 5% or more.

(Bcc) particles having a bcc (body center cubic) structure and a bct (body center square) structure (hereinafter collectively referred to as " bcc particles " The smaller the average particle diameter, the higher the ductility, work hardenability and stretch flangeability of the steel sheet having the low-temperature transformation forming phase as the main phase and the metal phase containing the retained austenite as the second phase. The reason for this is not clear, but it is presumed that (a) the arrangement of the retained austenite is suitably adjusted by the refinement of the bcc particles, and (b) the elongation of cracks is suppressed by grain refinement of the bcc particles .

 From the above results, it can be understood from the above results that the steel containing a certain amount or more of Si is hot rolled after raising the final reduction amount, quenched immediately thereafter, wound in a coil shape at a high temperature, cold rolled, annealed at a high temperature, And the second phase further comprises retained austenite and preferably polygonal ferrite and having fewer coarse austenite grains having a grain size of 1.2 탆 or more and preferably having a metal structure in which the bcc grains are fine, , A cold-rolled steel sheet excellent in work-hardening properties and stretch flangeability can be produced.

The steel sheet according to the present invention comprises, by mass%, C: more than 0.020% to less than 0.30%, Si: more than 0.10% to 3.00%, Mn: more than 1.00% to 3.50% : 0 to 2.00%, N: not more than 0.010%, Ti: not less than 0.050%, Nb: not less than 0% and not more than 0.050%, V: not less than 0% and not more than 0.50% Mo: 0% or more and 0.50% or less, B: 0% or more and 0.010% or less, Ca: 0% or more and 0.010% or less, Mg: 0% or more and 0.010% or less, REM: 0% or more and 0.050% 0.050% or less, and the remainder being Fe and impurities, wherein the main phase is a low-temperature transformation-generated phase and the second phase has a metal structure containing residual austenite, and the retained austenite has a to volume ratio is less than 4.0% greater than 25.0%, and the average particle diameter is less than 0.80μm, the retained austenite wherein the residual austenite grain diameter is less than 1.2μm number density 3.0 × 10 ^-2 / ㎛ ² is a cold-rolled steel sheet characterized in that not more than.

The metal structure of the cold-rolled steel sheet according to the present invention preferably satisfies one or both of the following:

Particles having a bcc structure and particles having a bct structure surrounded by a grain boundary of an azimuth angle of 15 ° or more are 7.0 μm or less;

The second phase contains residual austenite and polygonal ferrite, and the polygonal ferrite has a volume ratio of more than 2.0% to less than 27.0% with respect to the whole structure and an average particle size of less than 5.0 m.

In a preferred embodiment, the chemical composition further comprises at least one of the following elements (% is the total mass%):

At least one selected from the group consisting of Ti: at least 0.005% and less than 0.050%, Nb: at least 0.005% and less than 0.050%, and V: at least 0.010% and 0.50%

At least one selected from the group consisting of Cr: at least 0.20% and not more than 1.0%, Mo: at least 0.05% and not more than 0.50%, and B: at least 0.0010% and not more than 0.010%

At least one selected from the group consisting of Ca: 0.0005% to 0.010%, Mg: 0.0005% to 0.010%, REM: 0.0005% to 0.050%, and Bi: 0.0010% to 0.050%

According to the present invention, it is possible to obtain a high-strength cold-rolled steel sheet having sufficient ductility, work hardenability and stretch flangeability applicable to processing such as press forming. Therefore, the present invention contributes to the development of the industry, such as contributing to the solution of the global environmental problem by reducing the weight of the vehicle body.

1 is a graph showing the particle size distribution of retained austenite in a annealed steel sheet produced through a quenching process immediately after the quenching process.
Fig. 2 is a graph showing the particle size distribution of retained austenite in the annealed steel sheet produced immediately after the quenching process. Fig.
Figure 3 is a graph showing the relation between TS × λ ^{1 .7} and retained austenite grain size number density (N _R) of 1.2μm or more nitro.
4 is a graph showing the relationship between the value of TS x n and the number density (N _R ) of retained austenite having a grain size of 1.2 μm or more.

The metal structure, chemical composition and rolling, annealing conditions and the like in a production method capable of efficiently, stably, and economically manufacturing the steel sheet of the high-strength cold-rolled steel sheet according to the present invention will be described in detail below.

1. Metal structure

The cold-rolled steel sheet of the present invention is characterized in that the main phase is a low temperature transformation phase, and the second phase contains retained austenite and preferably polygonal ferrite. The retained austenite has a volume ratio of more than 4.0% to 25.0% and less than the average particle diameter is less than 0.80μm, the retained austenite of, and particle size of 1.2μm or more residual austenite number density is 3.0 × 10 ^-2 gae / ㎛ particles of ² or less, preferably grain boundaries over primary defense 15˚ Wherein the average particle diameter of the particles having the bcc structure and the bct structure is not more than 7.0 μm and / or the volume ratio of the polygonal ferrite to the whole structure is more than 2.0% and less than 27.0%, and the average particle diameter is less than 5.0 μm Organization.

The main phase means an image or a structure having the maximum volume ratio, and the second phase means a phase and a structure other than the main phase.

The low temperature transformation phase refers to phases and structures produced by low temperature transformation such as martensite or bainite. Examples of the low temperature transformation phase other than these are bainitic ferrite and trimmatiteite. Bainitic ferrite is distinguished from polygonal ferrite in that it has a lasic or plate-like morphology and a high dislocation density, and is distinguished from bainite in that iron carbide is not present in the inside and the interface.

The low-temperature transformation forming phase may include two or more phases and structures, for example, martensite and bainitic ferrite. In the case where the low temperature transformation product phase contains two or more phases and / or a structure, the total volume ratio of these phases and / or the structure is defined as the volume fraction of the low temperature transformation product phase.

The bcc phase is an image having a crystal structure of a body-centered cubic lattice type, and examples thereof include polygonal ferrite, bainitic ferrite, bainite and trimmatite. On the other hand, the bct phase is an image having a crystal structure of a body-centered tetragonal lattice (bct) type, and martensite is exemplified. The particles having the bcc structure are regions surrounded by a boundary of 15 degrees or more in the bcc phase. Similarly, a particle having a bct structure is a region surrounded by a boundary having an azimuth difference of 15 degrees or more in the bct phase. Hereinafter, the bcc phase and the bct phase are collectively referred to as the bcc phase. This is because, as will be described later, since the lattice constant is not taken into account in the evaluation of the metal structure by EBSP, the bcc phase and the bct phase are detected irrevocably.

This is because the main phase is a low-temperature transformation forming phase and the second phase contains residual austenite because it is suitable for improving ductility, work hardenability and stretch flangeability while maintaining tensile strength. If the main phase is a polygonal ferrite other than a low temperature transformation phase, securing of tensile strength and stretch flangeability becomes difficult.

The volume ratio of the retained austenite to the whole structure is set to be more than 4.0% and less than 25.0%. If the volume percentage of the retained austenite to the entire structure is 4.0% or less, the ductility becomes insufficient. Therefore, the volume ratio of the retained austenite to the entire structure is set to be more than 4.0%. , Preferably more than 6.0%, more preferably more than 9.0%, particularly preferably more than 12.0%. On the other hand, when the volume percentage of the retained austenite with respect to the entire structure is 25.0% or more, deterioration of stretch flangeability becomes remarkable. Therefore, the volume ratio of the retained austenite to the entire structure is made less than 25.0%. , Preferably less than 18.0%, more preferably less than 16.0%, particularly preferably less than 14.0%.

The average grain size of the retained austenite is set to be less than 0.80 mu m. In the cold-rolled steel sheet having the low-temperature transformation forming phase as the main phase and the metal phase containing the residual austenite in the second phase, when the average grain size of the retained austenite is 0.80 m or more, the ductility, work hardenability and stretch flangeability are significantly deteriorated do. The average grain size of the retained austenite is preferably less than 0.70 mu m, more preferably less than 0.60 mu m. The lower limit of the average grain size of the retained austenite is not particularly limited. However, in order to make the grain size smaller than 0.15 탆, the final rolling reduction of the hot rolling is required to be extremely high, and the production load is remarkably increased. Therefore, the lower limit of the average grain size of the retained austenite is preferably set to be more than 0.15 m.

In the cold-rolled steel sheet having the low-temperature transformation forming phase as the main phase and the metallic phase containing the residual austenite in the second phase, even if the average grain size of the retained austenite is less than 0.80 mu m, the coarse retained austenite grains The work hardenability and stretch flangeability are impaired. Therefore, the number density of retained austenite grains having a grain size of 1.2 탆 or more is 3.0 x 10 ^-2 pieces / 탆 ² or less. The residual austenite grain size number density of particles above 1.2μm nitro is 2.0 × 10 ^-2 gae / ㎛ ² or less is preferable, and if 1.5 × 10 ^-2 gae / ㎛ ² or less, and more preferably, 1.0 × 10 ^-2 gae / ㎛ ² Or less.

In order to further improve ductility and work hardenability, the second phase preferably contains polygonal ferrite in addition to retained austenite. It is preferable that the volume percentage of the polygonal ferrite with respect to the entire structure is more than 2.0%. , More preferably more than 8.0%, particularly preferably more than 13.0%. On the other hand, if the volume percentage of polygonal ferrite is excessive, stretch flangeability deteriorates. Therefore, the volume percentage of polygonal ferrite is preferably less than 27.0%. , More preferably less than 24.0%, particularly preferably less than 18.0%.

Further, as the fine grain of polygonal ferrite increases the effect of improving ductility and work hardening property, the average grain size of polygonal ferrite is preferably less than 5.0 m. More preferably less than 4.0 mu m, particularly preferably less than 3.0 mu m.

In order to further improve stretch flangeability, it is preferable that the volume percentage of temper martensite included in the low temperature transformation production phase is less than 50.0% with respect to the whole structure. , More preferably less than 35.0%, particularly preferably less than 10.0%.

In order to increase the tensile strength, it is preferable that the low temperature transformation forming phase includes martensite. In this case, it is preferable that the volume ratio of the martensite to the whole texture is more than 4.0%. , More preferably more than 6.0%, particularly preferably more than 10.0%. On the other hand, when the volume ratio of martensite is excessive, the stretch flangeability deteriorates. As a result, the volume percentage of martensite in the entire structure is preferably less than 15.0%.

In order to further improve ductility, work hardenability and stretch flangeability, the average of bcc particles (bcc particles mean collectively the particles having the bcc structure and the bct structure surrounded by the grain boundaries of 15 degrees or more in the orientation difference) The particle size is preferably 7.0 탆 or less. The average particle diameter of the bcc particles is more preferably 6.0 탆 or less, and particularly preferably 5.0 탆 or less.

The metal structure of the cold-rolled steel sheet according to the present invention is measured as follows. Namely, the volume ratio of the low-temperature transformation forming phase and the polygonal ferrite is determined by taking a test piece from a steel sheet, polishing a longitudinal section parallel to the rolling direction, And the area ratio of the polygonal ferrite is measured by image processing. The area ratio is determined to be equal to the volume ratio, and the respective volume ratios are obtained. The average particle size of the polygonal ferrite is determined by dividing the area occupied by the entire polygonal ferrite in the field of view by the number of crystal grains of the polygonal ferrite and calculating the circle equivalent diameter.

The volume percentage of retained austenite is obtained by measuring the X-ray diffraction intensity using XRD by collecting a test piece from the steel sheet, chemically polishing the rolled surface from the surface of the steel sheet to the 1/4 depth of the plate thickness.

The particle size of the retained austenite particles and the average particle size of the retained austenite are measured as follows. That is, a test piece is taken from the steel sheet, the longitudinal section parallel to the rolling direction is electrolytically polished, and the metal structure is observed using a SEM equipped with EBSP at 1/4 depth of the plate thickness from the surface of the steel sheet. (Fcc phase) composed of a face-centered cubic crystal structure, and the region surrounded by the parent phase is regarded as one residual austenite particle, and the number density of the retained austenite particles (the number of particles per unit area) and The area ratio of the individual retained austenite particles is measured. The circle equivalent diameters of the individual austenite grains are determined from the areas occupied by the individual retained austenite grains in the visual field, and the average value thereof is taken as the average grain size of the retained austenite

In EBSP-based tissue observation, an electron beam is irradiated at an angle of 0.1 占 퐉 in an area of 50 占 퐉 or more in the sheet thickness direction and 100 占 퐉 or more in the rolling direction. Among the obtained measurement data, a Confidence Index of 0.1 or more is used as effective data for particle size measurement. In order to prevent the grain size of the retained austenite from being underestimated due to the measurement noise, only the retained austenite grains having a circle equivalent diameter of 0.15 占 퐉 or more are used as effective grains to calculate the average grain size.

The average particle diameter of bcc particles is measured as follows. That is, a test piece is taken from the steel sheet, the longitudinal section parallel to the rolling direction is electrolytically polished, and the metal structure is observed using a SEM equipped with EBSP at 1/4 depth of the plate thickness from the surface of the steel sheet. The area surrounded by the boundary of 15 degrees or more is regarded as one bcc particle, and the value calculated according to the definition of the following formula (1) is taken as the average particle diameter of the bcc particle. Where N is the number of crystal grains contained in the average grain size evaluation area, A _i is the area of the crystal grains of the i-th (i = 1, 2, ..., N), d _i is the circle- Respectively.

[Number 1]

In the present invention, particles having a bcc structure and particles having a bct structure are treated as one body. This is because the evaluation of the metal structure by EBSP does not take into account the lattice constant. Therefore, particles having a bcc structure (for example, polygonal ferrite, bainitic ferrite, bainite, and tempered martensite) For example, martensite).

At this time, also in the tissue observation by EBSP, the electron beam is irradiated at an angle of 0.1 占 퐉 in the region of the size of 50 占 퐉 in the plate thickness direction and 100 占 퐉 in the rolling direction, as described above. Among the obtained measurement data, a reliability index of 0.1 or more is used as effective data for particle size measurement. Further, in order to prevent underestimation of the particle size due to the measurement noise, in the evaluation of the bcc phase, the grain diameter is calculated by using only bcc particles having particle diameters of 0.47 μm or more as effective particles, unlike the above-described residual austenite. In the case where the texture is a mixed grain structure in which fine grains and coarse grains are mixed, the influence of the coarse grains may be underestimated in an evaluation by a cutting method generally used as an evaluation of crystal grain size of a metal structure. In the present invention, as the calculation method of the crystal grain size in consideration of the influence of coarse particles, the above formula (1) in which the area of each crystal grain is measured as the weight is used.

In the present invention, in the case of the cold-rolled steel sheet, at the 1/4 depth of the sheet thickness from the steel sheet surface, in the case of the coated steel sheet, from the boundary between the steel sheet and the plating layer, Describe the metallurgical structure of the description.

 As a mechanical property that can be realized based on the above-described features of the metallographic structure, the cold-rolled steel sheet according to the present invention has a tensile strength (TS) of 780 MPa or more in the direction perpendicular to the rolling direction And more preferably 950 MPa or higher. On the other hand, in order to secure ductility, TS is preferably less than 1180 MPa.

From the viewpoint of press formability, the total elongation (El ₀ ) in the direction perpendicular to the rolling direction is calculated in accordance with El, JIS Z2253 of Japanese Industrial Standards The n-value, which is the work hardening index calculated using the nominal deformation at two points of 5% and 10% with the deformation range of 5% to 10% and the corresponding test force, and the value measured according to the Japan Steel Federation standard JFST1001 For a hole expansion factor of lambda,

The value of TS x El is 19000 MPa% or more, particularly 20000 MPa or more,

· A value of more than the value of TS × n value of 160MPa, 165MPa or more in particular, and TS × λ is not less than ^{1 .7} 5500000MPa than ^1.7%, particularly 6000000MPa ^{1 .7%.}

_{El = El 0 × (1.2 /} t 0) 0.2 ··· (2)

El ₀ in the equation represents the measured value of the total elongation measured using the JIS No. 5 tensile test specimen and t ₀ represents the plate thickness of the JIS No. 5 tensile specimen provided for the measurement, Is the equivalent value of the total elongation.

The work hardening index is represented by an n value with respect to a strain range of 5 to 10% in a tensile test, since deformation occurring when an automobile part is press molded is about 5 to 10%. Even if the total elongation of the steel sheet is high, when the value of n is low, deformation propagation property becomes insufficient in the press forming of an automobile part, and molding defects such as local plate thickness reduction are likely to occur. From the viewpoint of shape freezing property, the yield ratio is preferably less than 80%, more preferably less than 75%, and particularly preferably less than 70%.

2. Chemical composition of steel

C: more than 0.020% and less than 0.30%

When the C content is 0.020% or less, it is difficult to obtain the above-mentioned metal structure. Therefore, the C content should be more than 0.020%. , Preferably more than 0.070%, more preferably more than 0.10%, particularly preferably more than 0.14%. On the other hand, when the C content is 0.30% or more, not only the stretch flangeability of the steel sheet is deteriorated but also the weldability is deteriorated. Therefore, the C content should be less than 0.30%. , Preferably less than 0.25%, more preferably less than 0.20%, particularly preferably less than 0.17%.

Si: more than 0.10% and not more than 3.00%

Si has an action to improve ductility, work hardenability and stretch flangeability through inhibition of austenite grain growth during annealing. It also has an effect of enhancing the stability of austenite and is an element effective for obtaining the above-mentioned metal structure. When the Si content is 0.10% or less, it is difficult to obtain the effect of the above action. Therefore, the Si content should be more than 0.10%. , Preferably more than 0.60%, more preferably more than 0.90%, particularly preferably more than 1.20%. On the other hand, when the Si content exceeds 3.00%, the surface properties of the steel sheet deteriorate. In addition, the chemical conversion treatment and the plating ability are remarkably deteriorated. Therefore, the Si content is set to 3.00% or less. , Preferably less than 2.00%, more preferably less than 1.80%, particularly preferably less than 1.60%.

In the case of containing Al to be described later, it is preferable that the Si content and the sol.Al content satisfy the following formula (3), more preferably satisfy the following formula (4) desirable.

Si + sol.Al > 0.60 (3)

Si + sol.Al > 0.90 (4)

Si + sol.Al > 1.20 (5)

Here, Si in the formula represents the Si content in the steel, and sol.Al represents the acid-soluble Al content in mass%.

Mn: more than 1.00% and less than 3.50%

Mn has an effect of improving the hardenability of steel, and is an element effective for obtaining the above-mentioned metal structure. When the Mn content is 1.00% or less, it is difficult to obtain the above-mentioned metal structure. Therefore, the Mn content should be more than 1.00%. , Preferably more than 1.50%, more preferably more than 1.80%, particularly preferably more than 2.10%. If the Mn content is excessive, a coarse low-temperature transformation forming phase occurs in the rolling direction in the metal structure of the hot-rolled steel sheet, the coarse retained austenite grains increase in the metal structure after cold rolling and annealing, And stretch flangeability deteriorates. Therefore, the Mn content should be 3.50% or less. , Preferably less than 3.00%, more preferably less than 2.80%, particularly preferably less than 2.60%.

P: not more than 0.10%

P is an element contained in the steel as an impurity and is segregated at grain boundaries to brittle the steel. Therefore, the smaller the P content, the better. Therefore, the P content should be 0.10% or less. , Preferably less than 0.050%, more preferably less than 0.020%, particularly preferably less than 0.015%.

S: not more than 0.010%

S is an element contained in the steel as an impurity and forms sulfide inclusions to deteriorate elongation flangeability. Therefore, the smaller the S content is, the better. Therefore, the S content should be 0.010% or less. , Preferably less than 0.005%, more preferably less than 0.003%, particularly preferably less than 0.002%.

sol.Al: less than 2.00%

Al has an action of deoxidizing molten steel. In the present invention, since Al having a deoxidizing action is contained in the same manner as Al, Al is not necessarily contained. That is, it may be as close to 0% as possible. When it is contained for the purpose of accelerating deoxidation, it is preferable that it is contained in an amount of 0.0050% or more as sol.Al. A more preferred sol.Al content is greater than 0.020%. Al also has an effect of enhancing the stability of austenite like Si, and is an element effective for obtaining the above-mentioned metal structure, and thus Al can also be contained for this purpose. In this case, the sol.Al content is preferably more than 0.040%, more preferably more than 0.050%, particularly preferably more than 0.060%.

On the other hand, if the content of sol.Al is too high, not only surface scratches due to alumina tend to occur, but also the transformation point is greatly increased, making it difficult to obtain a metal structure having a low-temperature transformation forming phase as a main phase. Therefore, the content of sol.Al should be 2.00% or less. , Preferably less than 0.60%, more preferably less than 0.20%, particularly preferably less than 0.10%.

N: 0.010% or less

N is an element contained in the steel as an impurity and deteriorates ductility. Therefore, the smaller the N content, the better. Therefore, the N content should be 0.010% or less. It is preferably not more than 0.006%, more preferably not more than 0.005%.

The steel sheet according to the present invention may contain, as an arbitrary element, an element to be described below.

At least one selected from the group consisting of Ti: less than 0.050%, Nb: less than 0.050%, and V: 0.50%

Ti, Nb and V have the function of increasing the work strain by suppressing recrystallization in the hot rolling step and finely reducing the metal structure of the hot-rolled steel sheet. Further, it has a function of precipitating as carbide or nitride and suppressing coarsening of austenite during annealing. Therefore, one or more of these elements may be contained. However, even if it is contained in excess, the effect due to the action is saturated, which is not economical. In addition, the recrystallization temperature at the time of annealing increases, the metal structure after annealing becomes uneven, and the stretch flangeability is also damaged. Further, the deposition amount of carbide or nitride increases, yield ratio increases, and shape freezing property also deteriorates.

Therefore, the Ti content is less than 0.050%, the Nb content is less than 0.050%, and the V content is not more than 0.50%. The Ti content is preferably less than 0.040%, more preferably less than 0.030%, the Nb content is preferably less than 0.040%, more preferably less than 0.030%, the V content is preferably not more than 0.30% Preferably less than 0.050%. In order to more reliably obtain the effect of the above action, it is preferable that at least one of Ti: 0.005% or more, Nb: 0.005% or more, and V: 0.010% or more is satisfied. When Ti is contained, the Ti content is more preferably 0.010% or more. When Nb is contained, the Nb content is more preferably 0.010% or more. When V is contained, More preferably 0.020% or more.

Not more than 1.0% of Cr, not more than 0.50% of Mo, and not more than 0.010% of B,

Cr, Mo and B have an effect of improving the hardenability of steel, and are effective elements for obtaining the above-mentioned metal structure. Therefore, one or more of these elements may be contained. However, even if it is contained in excess, the effect due to the action is saturated, which is not economical. Therefore, the Cr content is 1.0% or less, the Mo content is 0.50% or less, and the B content is 0.010% or less. The Cr content is preferably 0.50% or less, the Mo content is preferably 0.20% or less, and the B content is preferably 0.0030% or less. In order to more reliably obtain the effect of the above action, it is preferable that Cr: 0.20% or more, Mo: 0.05% or more, and B: 0.0010% or more are satisfied.

At least one selected from the group consisting of Ca: not more than 0.010%, Mg: not more than 0.010%, REM: not more than 0.050%, and Bi: not more than 0.050%

Ca, Mg, and REM have the function of adjusting the shape of the inclusions so that Bi improves the elongation flangeability by making the solidification structure finer. Therefore, one or more of these elements may be contained. However, even if it is contained in excess, the effect due to the action is saturated, which is not economical.

Therefore, the Ca content is 0.010% or less, the Mg content is 0.010% or less, the REM content is 0.050% or less, and the Bi content is 0.050% or less. Preferably, the Ca content is 0.0020% or less, the Mg content is 0.0020% or less, the REM content is 0.0020% or less, and the Bi content is 0.010% or less. In order to more surely obtain the above action, it is preferable that Ca: 0.0005% or more, Mg: 0.0005% or more, REM: 0.0005% or more and Bi: 0.0010% or more are satisfied. REM means a rare earth element, and is a generic name of a total of 17 elements of Sc, Y and lanthanoid, and the REM content is the total content of these elements.

3. Manufacturing conditions

The steel having the above-described chemical composition is made into a steel ingot by a continuous casting method after being melted by a known means, or a steel ingot is formed into a steel ingot by an arbitrary casting method and then a steel ingot is rolled. In the continuous casting step, in order to suppress the occurrence of surface defects caused by inclusions, it is preferable to generate an external additional flow in the molten steel such as electron stirring in the mold. The steel ingot or the steel strip may be reheated once and then supplied to the hot rolling. Alternatively, the steel ingot in the high temperature state after the continuous casting or the high temperature state after the granite rolling may be maintained in this state, Or may be provided for hot rolling. In this specification, such ingot and steel strip are collectively referred to as " slab " The temperature of the slab to be provided in the hot rolling is preferably less than 1250 占 폚 and more preferably not higher than 1200 占 폚 in order to prevent coarsening of austenite. The lower limit of the temperature of the slab to be provided to the hot rolling is not particularly limited, but may be a temperature at which the hot rolling can be completed at an Ar ₃ point or more as described later.

The hot rolling is completed in a temperature range of Ar ₃ points or more in order to miniaturize the metal structure of the hot-rolled steel sheet by transforming austenite after completion of rolling. If the temperature at which the rolling is completed is too low, a coarse low temperature transformation forming phase occurs in the rolling direction in the metal structure of the hot-rolled steel sheet. The coarse retained austenite grains increase in the metal structure after cold rolling and annealing, and the work hardenability and stretch flangeability are likely to deteriorate. For this reason, it is preferable that the completion temperature of the hot rolling is not less than ₃ points and not less than 820 degrees centigrade. More preferably, the Ar ₃ point or more is also more than 850 ° C, particularly preferably the Ar ₃ point or more is also more than 880 ° C. On the other hand, if the temperature at which the rolling is completed is too high, accumulation of processing deformation becomes insufficient and it becomes difficult to make the metal structure of the hot-rolled steel sheet finer. Therefore, the completion temperature of hot rolling is preferably less than 950 占 폚, and more preferably less than 920 占 폚. Further, in order to reduce the production load, it is preferable to increase the completion temperature of hot rolling to lower the rolling load. From this point of view, it is preferable that the completion temperature of the hot rolling is higher than Ar ₃ point and higher than 780 ° C, and more preferably higher than Ar ₃ point and higher than 800 ° C.

Further, in the case where the hot rolling comprises the rough rolling and the finishing rolling, the controlled-pressure expanding material may be heated between the rough rolling and the finishing rolling to finish the finish rolling at the above-mentioned temperature. At this time, it is preferable to control the temperature fluctuation to 140 占 폚 or less over the entire length of the pressure control strip at the start of the finish rolling by heating the rear end of the pressure control strip to be higher than the front end. This improves the uniformity of the product characteristics in the coil.

The heating method of the pressure control strip may be carried out by a known means. For example, a solenoid-type induction heating apparatus may be provided between the rough rolling mill and the finish rolling mill, and the heating temperature rise amount may be controlled based on the temperature distribution in the lengthwise direction of the pressurized softening material on the upstream side of the induction heating apparatus.

The reduction amount of the hot rolling is such that the reduction amount of the final one pass exceeds 25% in terms of plate thickness reduction rate. This is because, in order to refine the metal structure of the hot-rolled steel sheet by increasing the amount of work deformation introduced into the austenite, to suppress the formation of coarse retained austenite grains in the metal structure after cold rolling and annealing, to be. When the second phase contains polygonal ferrite, the polygonal ferrite is to be refined. The reduction amount of the final one pass is preferably more than 30%, more preferably more than 40%. If the reduction amount is too high, the rolling load increases and rolling becomes difficult. Therefore, the reduction amount of the final one pass is preferably less than 55%, more preferably less than 50%. In order to lower the rolling load, so-called lubrication rolling may be performed in which rolling oil is supplied between the rolling roll and the steel plate to decrease the friction coefficient and then rolling.

After hot rolling, rapid cooling is carried out up to a temperature of 720 ° C or less within 0.40 seconds after completion of rolling. This is because the release of the work strain introduced into the austenite by the rolling is suppressed, the austenite is transformed as a driving force by the working deformation, the metal structure of the hot-rolled steel sheet is made finer, This is to inhibit the formation of austenite grains and to grain the bcc particles. When the second phase contains polygonal ferrite, the polygonal ferrite is to be refined. Preferably, the steel sheet is quenched to a temperature of 720 占 폚 or less within 0.30 seconds after completion of rolling, more preferably quenched to a temperature of 720 占 폚 or less within 0.20 seconds after completion of rolling. Since the release of the working deformation is suppressed as the average cooling rate during quenching is increased, the average cooling rate during quenching is preferably set to 300 ° C / s or more, whereby the metal structure of the hot-rolled steel sheet can be further miniaturized . It is more preferable to set the average cooling rate during quenching to 400 DEG C / s or more, and particularly preferable to be 600 DEG C / s or more. The time from the completion of rolling to the start of quenching and the cooling rate therebetween need not be specially specified.

A water spraying apparatus having a high water density is preferably used. A water spray header is disposed between the rolling plate conveying rollers, and a water spray header is disposed between the rolling plate conveying rollers. A method of spraying high-pressure water is exemplified.

After quenching, the steel sheet is rolled in a temperature range exceeding 500 ° C. This is because if the coiling temperature is 500 캜 or less, iron carbide does not sufficiently precipitate in the hot-rolled steel sheet, coarse retained austenite grains are formed in the metal structure after cold rolling and annealing, and bcc grains are assembled . The coiling temperature is preferably more than 550 DEG C, more preferably 580 DEG C or more. On the other hand, if the coiling temperature is too high, ferrite becomes coarse in the hot-rolled steel sheet, and coarse retained austenite grains are produced in the metal structure after cold rolling and annealing. Therefore, the coiling temperature is preferably less than 650 占 폚, and more preferably less than 620 占 폚.

The conditions from the quenching stop to the coiling are not particularly specified, but it is preferable that the quenching is stopped for at least 1 second in the temperature range of 720 to 600 占 폚 after quenching. As a result, the generation of fine ferrite is promoted. On the other hand, if the holding time is too long, the productivity is impaired. Therefore, it is preferable to set the upper limit of the holding time within the range of 720 to 600 ° C within 10 seconds. After holding at a temperature range of 720 to 600 占 폚, it is preferable to cool the coiling up to a coiling temperature at a cooling rate of 20 占 폚 / s or more in order to prevent coarsening of the generated ferrite.

The hot-rolled steel sheet is descaled by pickling or the like, and then cold-rolled according to a conventional method. In cold rolling, it is preferable to set the cold pressing rate (total rolling reduction in cold rolling) to 40% or more in order to accelerate recrystallization to homogenize the metal structure after cold rolling and annealing and to further improve elongation flangeability. If the cold pressing rate is too high, the rolling load becomes large and rolling becomes difficult, so the upper limit of the cold pressing rate is preferably less than 70%, more preferably less than 60%.

The steel sheet after cold rolling is subjected to degreasing or the like according to a known method, if necessary, and then annealed. The lower limit of the cracking temperature in annealing is set to (Ac ₃ point -40 ° C) or higher. This is to obtain a metal structure in which the main phase is a low-temperature transformation forming phase and the second phase contains retained austenite. In order to increase the volume ratio of the low-temperature transformation forming phase and to improve the elongation flangeability, the cracking temperature is preferably higher than (Ac ₃ point -20 ° C), more preferably Ac ₃ point higher. However, if the cracking temperature becomes too high, austenite becomes excessively coarse, the metal structure after annealing coarsens, and generation of polygonal ferrite is suppressed, resulting in deterioration of ductility, work hardenability and stretch flangeability. Therefore, the upper limit of the soaking temperature is, particularly preferred, and it is more preferable as it is less than desirable and, (Ac ₃ point + 50 ℃) that is less than (Ac ₃ point + 100 ℃), which is less than (Ac ₃ point + 20 ℃) Do. By making the upper limit of the crack temperature lower than (Ac ₃ point + 50 ° C), the bcc particles can be finely granulated to an average particle diameter of 7.0 μm or less, and particularly excellent ductility, work hardenability and stretch flangeability can be obtained.

The holding time (cracking time) at the cracking temperature is not particularly limited, but is preferably more than 15 seconds, more preferably more than 60 seconds, in order to obtain stable mechanical characteristics. On the other hand, if the holding time becomes too long, the austenite becomes excessively coarse, and ductility, work hardening property and stretch flangeability are liable to deteriorate. Therefore, the holding time is preferably less than 150 seconds, and more preferably less than 120 seconds.

In the heating process in the annealing, it is preferable to set the heating rate from 700 占 폚 to the cracking temperature to less than 10.0 占 폚 / s in order to facilitate recrystallization, uniformize the metal structure after annealing, and further improve elongation flangeability. More preferably less than 8.0 DEG C / s, and particularly preferably less than 5.0 DEG C / s.

In the cooling process after the cracking in the annealing, it is preferable to cool the cracking temperature by 50 占 폚 or more from the cracking temperature at a cooling rate of less than 5.0 占 폚 / s in order to promote the generation of fine polygonal ferrite and improve ductility and work hardening property. The cooling rate at this time is more preferably less than 3.0 DEG C / s, and particularly preferably less than 2.0 DEG C / s. In order to further increase the volume percentage of polygonal ferrite, it is more preferable to cool at 80 ° C or more, particularly preferably at 100 ° C or more, and most preferably at 120 ° C or more. (Ac ₃ point + 50 ° C), and then cooled to 50 ° C or more from the cracking temperature at a cooling rate of less than 5.0 ° C / s to obtain a polygonal ferrite having an average particle size of less than 5.0 μm as a volume ratio of 2.0% And it is possible to obtain excellent ductility, work hardening property and stretch flangeability.

It is preferable to cool the temperature range of 650 to 500 占 폚 at a cooling rate of 15 占 폚 / s or more in order to obtain a metal structure having a low-temperature transformation forming phase as a main phase. It is more preferable to cool the temperature range of 650 to 450 DEG C at a cooling rate of 15 DEG C / s or more. The higher the cooling rate is, the higher the volume ratio of the low-temperature transformation forming phase becomes. Therefore, the cooling rate is more preferably 30 ° C / s and more preferably 50 ° C / s in any of the above temperature ranges. On the other hand, if the cooling rate is too high, the shape of the steel sheet is damaged. Therefore, it is preferable that the cooling rate in the temperature range of 650 to 500 占 폚 is 200 占 폚 / s or less. More preferably less than 150 ° C / s, and particularly preferably less than 130 ° C / s.

In order to secure the retained austenite amount, it is maintained at a temperature range of 450 to 340 ° C. for 30 seconds or more during the cooling process. In order to enhance the stability of retained austenite and further improve ductility, work hardenability and stretch flangeability, it is preferable to set the holding temperature range to 430 to 360 ° C. The longer the holding time, the higher the stability of the retained austenite. Therefore, the holding time is preferably 60 seconds or longer. More preferably 120 seconds or more, and particularly preferably 300 seconds or more.

In the case of producing an electroplated steel sheet, the cold rolled steel sheet produced by the above-described method may be subjected to known pretreatment for cleaning and adjustment of the surface, if necessary, and then electroplated according to a conventional method. The composition and the amount of deposition are not limited. Examples of electroplating include electro-galvanizing, electro-Zn-Ni alloy plating and the like.

In the case of producing a hot-dip coated steel sheet, the annealing step is carried out by the above-described method, and the steel sheet is held at a temperature range of 450 to 340 DEG C for 30 seconds or longer. Thereafter, . In order to enhance the stability of retained austenite and further improve ductility, work hardenability and stretch flangeability, it is preferable to set the holding temperature range to 430 to 360 ° C. The longer the holding time, the higher the stability of the retained austenite. Therefore, the holding time is preferably 60 seconds or longer. More preferably 120 seconds or more, and particularly preferably 300 seconds or more. After the hot-dip coating, re-heating may be performed to perform the alloying treatment. The chemical composition and deposition amount of the plated film are not limited. Examples of the type of hot-dip coating include hot-dip galvanizing, hot-dip galvanizing, hot-dip galvanizing, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating and hot-dip Zn-Al-Mg-Si alloy plating.

The plated steel sheet may be subjected to a suitable chemical conversion treatment after plating in order to further increase its corrosion resistance. The chemical conversion treatment is preferably carried out using a non-chrome chemical conversion solution (for example, silicate, phosphate or the like) instead of the conventional chromate treatment.

The cold-rolled steel sheet and the coated steel sheet thus obtained may be subjected to temper rolling according to a conventional method. However, a high elongation percentage of the temper rolling causes deterioration of ductility, so that the elongation of the temper rolling is preferably 1.0% or less. A more preferable elongation is 0.5% or less.

 The present invention is illustrated by the following examples. The present invention is not limited by these Examples.

Example 1

Using a laboratory vacuum melting furnace, the steel having the chemical composition shown in Table 1 was melted and cast. Each of the obtained ingot was made into a 30 mm-thick piece of steel by hot forging. The slabs were heated to 1200 DEG C using an electric heating furnace, held at this temperature for 60 minutes, and then hot-rolled under the conditions shown in Table 2. [

Concretely, 6 passes were performed at a temperature range of Ar ₃ points or more using an experimental hot rolling mill, and the thickness was 2 to 3 mm. The reduction rate of the final one pass was set to 12 to 42% in terms of plate thickness reduction rate. After hot rolling, the steel sheet is cooled to 650 to 720 占 폚 in various cooling conditions using a water spray, allowed to cool for 5 to 10 seconds, cooled to various temperatures at a cooling rate of 60 占 폚 / s, , Charged into an electric heating furnace maintained at the same temperature, held for 30 minutes, cooled to room temperature at a cooling rate of 20 DEG C / h, and simulated annealing after winding was performed to obtain a hot-rolled steel sheet.

The obtained hot-rolled steel sheet was pickled to obtain a cold-rolled base material, and cold-rolled at a cold pressing rate of 50 to 60% to obtain a cold-rolled steel sheet having a thickness of 1.0 to 1.2 mm. The obtained cold-rolled steel sheet was heated to 550 DEG C at a heating rate of 10 DEG C / s, heated to various temperatures shown in Table 2 at a heating rate of 2 DEG C / s, and cracked for 95 seconds . Thereafter, primary cooling to the temperatures shown in Table 2 was carried out, secondary cooling was carried out from the primary cooling stop temperature to the various cooling stop temperatures shown in Table 2 at an average cooling rate of 60 占 폚 / s and maintained at that temperature for 330 seconds And then cooled to room temperature to obtain a annealed steel sheet.

[Table 1]

[Table 2]

The test specimen for SEM observation was taken from the annealed steel plate, the longitudinal section parallel to the rolling direction was polished, and the steel structure was corroded with a detaching process. The metal structure at the 1/4 depth of the plate thickness from the steel plate surface was observed, By the treatment, the low-temperature transformation production phase and the volume fraction of polygonal ferrite were measured. Further, the area occupied by the entire polygonal ferrite was divided by the number of crystal grains of polygonal ferrite, and the average grain size (circle equivalent diameter) of polygonal ferrite was obtained.

Further, a test piece for XRD measurement was sampled from the annealed steel sheet, and the rolled surface was chemically polished to a depth of 1/4 of the plate thickness from the surface of the steel sheet, and subjected to an X-ray diffraction test to measure the volume fraction of the retained austenite. Specifically, RINT2500 manufactured by Rigaku Corporation was used in the X-ray diffraction apparatus, and Co-K? Ray was incident to obtain α-phase (110), (200) and (211) diffraction peaks and γ- The integral intensity of the (220) diffraction peak was measured to determine the volume fraction of retained austenite.

Further, a test piece for EBSP measurement was taken from the annealed steel sheet, the longitudinal section parallel to the rolling direction was electrolytically polished, the metal structure was observed at the 1/4 depth of the plate thickness from the steel sheet surface, and bcc The average particle diameter of the particles, the particle diameter distribution of the residual austenite particles, and the average particle diameter of the retained austenite were measured. Specifically, TSL OIM5 was used for the EBSP measuring apparatus, and electron beams were irradiated at a pitch of 0.1 占 퐉 in a region of 50 占 퐉 in the plate thickness direction and 100 占 퐉 in the rolling direction, and a reliability index of 0.1 or more Bcc phase and fcc phase were judged as data.

and the area enclosed by the grain boundaries at an azimuth angle of 15 DEG or more is regarded as one bcc particle and the circle equivalent diameter and area of each individual bcc particle are determined and the average particle diameter is calculated according to the definition of the above- Respectively. In order to calculate the average particle diameter, bcc particles having a circle equivalent diameter of 0.47 탆 or more were made effective bcc particles. The crystal structure of the martensite is strictly a body center square lattice (bct), but since the lattice constant is not considered in the evaluation of the metal structure by EBSP, martensite is also treated as the bcc phase.

Also, as the fcc phase, the area surrounded by the parent phase was regarded as one retained austenite grain, and the circle equivalent diameter of each of the retained austenite grains was obtained. The average austenite size of the retained austenite was determined as the effective retained austenite grains having the circle equivalent diameter of 0.15 탆 or more as the effective retained austenite grains and the average value of the circle equivalent diameters of the individual effective retained austenite grains. Also, the number density (N _R ) per unit area of the retained austenite grains having a grain size of 1.2 탆 or more was determined.

The yield stress (YS) and the tensile strength (TS) were obtained by taking a JIS No. 5 tensile test specimen from the annealed steel sheet along the direction immediately preceding the rolling direction and performing a tensile test at a tensile speed of 10 mm / min. The overall elongation El was determined by tensile test with a JIS No. 5 tensile test specimen taken along the direction immediately preceding the rolling direction and using the measured actual value El _{0 and} calculating the sheet thickness And the converted value corresponding to the case of 1.2 mm was obtained. The work hardening index (n value) was determined by the tensile test with the JIS No. 5 tensile test specimen taken along the direction immediately preceding the rolling direction, and the strain range was 5 to 10%. Specifically, the test force for the nominal strain of 5% and 10% was used to calculate by the two-point method.

The stretch flangeability was evaluated by measuring the hole expanding rate (?) In the following manner. A 100 mm square plate was taken from the annealed steel plate, and a punching hole having a diameter of 10 mm was punched with a clearance of 12.5%. A punching hole was extended from the side punched with a conical punch having a tip angle of 60 to generate cracks penetrating the plate thickness The enlargement ratio of the hole was measured, and this was regarded as the hole expansion rate.

Table 3 shows the results of metal structure observation and performance evaluation of the cold-rolled steel sheet after annealing. In Table 1 to Table 3, numerals or symbols denoted by " * " mean those outside the scope of the present invention.

[Table 3]

Test results for the steel sheet in the range defined by the present invention are all, and the value of TS × El more than 19000MPa%, and the value of TS × n value of more than 160, the value of TS × λ ^{1 .7} more than ^1.7% 6000000MPa , And exhibited good ductility, work hardenability and stretch flangeability. And the second phase contains polygonal ferrite in addition to the retained austenite, the volume ratio of the polygonal ferrite is more than 2.0% and less than 27.0%, the average particle diameter is 5.0 m If less than, the value of the value of TS × El more than 20000MPa%, the value of TS × n value of more than 165, TS ^{1 .7} × λ is at least ^{1 .7%} 6000000MPa, ductility, the curing process and the stretch flangeability Further improvement.

Claims (6)

C: not less than 0.020%, not more than 0.30%, Si: not more than 0.10%, not more than 3.00%, Mn: not more than 1.00%, not more than 3.50%, P: not more than 0.10%, S: not more than 0.010% Ti: 0% or more and less than 0.050%, Nb: 0% or more and less than 0.050%, V: 0% or more and 0.50% or less, Cr: 0% or more and 1.0% or less, Mo: 0% 0% or more and 0.010% or less, B: 0% or more and 0.010% or less, Ca: 0% or more and 0.010% or less, Mg: 0% or more and 0.010% or less, REM: 0% or more and 0.050% And the balance of Fe and impurities,
Wherein the main phase has a low temperature transformation phase and the second phase has a metal structure containing retained austenite, wherein the retained austenite has a volume ratio of more than 4.0% to less than 25.0%, an average grain size of less than 0.80 mu m, the residual austenite of the, grain size of 1.2μm or more residual austenite grain number density is 3.0 × 10 ^-2 gae / μm ² the cold-rolled steel sheet excellent work-hardening properties, characterized in that not more than a. The method according to claim 1,
Wherein the average grain size of grains having a bcc structure and grains having a bct structure surrounded by grain boundaries of 15 degrees or more in azimuthal difference is 7.0 占 퐉 or less. The method according to claim 1 or 2,
Wherein the second phase contains residual austenite and polygonal ferrite, and the polygonal ferrite has a volume ratio of more than 2.0% to less than 27.0% and an average grain size of less than 5.0 占 퐉 with respect to the whole structure. The method according to claim 1 or 2,
Wherein said chemical composition contains at least one selected from the group consisting of Ti: at least 0.005% and less than 0.050%, Nb: at least 0.005% and less than 0.050%, and V: at least 0.010% and at most 0.50% , Cold rolled steel plate. The method according to claim 1 or 2,
Wherein the chemical composition is at least one selected from the group consisting of Cr: at least 0.20% to at most 1.0%, Mo: at least 0.05% and at most 0.50%, and B: at least 0.0010% to 0.010% , Cold rolled steel plate. The method according to claim 1 or 2,
Wherein the chemical composition is selected from the group consisting of Ca: 0.0005% to 0.010%, Mg: 0.0005% to 0.010%, REM: 0.0005% to 0.050%, and Bi: 0.0010% to 0.050% A cold-rolled steel sheet containing one or two or more thereof.

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